Enamel opacifier and method of use



Patented Jan. 16, 1951 UNITED STATES PATENT OFFICE ENAMEL OPACIFIER ANDMETHOD OF USE No Drawing. Application December 14, 1946, Serial No.716,429

opacity on heating in the enameling cycle.

In the enameling of metal and particularly sheet iron and steel, theultimate object is to obtain a vitreous coating upon the metal basewhich presents a high degree of opacity as this is an estheticrequirement in enameling, as great a coverage as possible and a finishedvitreous coating resistant to strain and impact incident to assembly,storage, transportation or use.

A very important feature of a finished enamel article and, therefore, ofthe enamel to be applied to the article, is the diffuse reflectance oropacity of the enamel coating. In general, the opacity may be obtainedby adding materials to the glass composing the frit as, for instance,certain compounds of fluorine or antimony, to mention but two of themore popular older types,

or zircon as a more recent opacifier. Or, opaciriers may be added to thefrit subsequent to its manufacture and milled therewith as a separateindividual ingredient, so-called mill addition cpacifiers.

Vitreous enamel coatings of exceptional properties have been produced inrecent years which depend for their opacity upon the presence ofcompounds of zirconium in the frit. Patent Nos. 1,944,938 of January 30,1934, to C. J. Kinzie and 2,326,343 of August 10, 1943, to Frost andCommons relate to enameling or glazing frit compositions depending uponzircon type opacifiers to obtain relatively high reflectance values;additional phases of this important fleld are described in patents toBahnsen et al. Nos. 2,250,456 and 2,250,457 of July 29, 1941 and2,324,812 of July 20, 1943. Frits of this type develop opacity in theenameling cycle, in contrast, with frits of other types wherein theopacity is already developed in the manufacture of the frit.

In general, the greater the amount of an opacifier in a frit, the higherthe opacity of the finished enamel; however, it is known that, at times,surface dimculties in the enamel may be caused by attempts to includetoo great an amount of an opacifier in the frit glass. These surfacedifficulties decrease the esthetic value of the enamel coating and,therefore, in eiiect, defeat one of the purposes of the addition of theopacifier.

In accordance with the present invention the opacity of zirconiumopacified irits of the type 2 Claims. (Cl. 10648) 2 which developopacity on heating the frit during the enameling cycle may have theiropacity increased by the employment of specially pre-- pared opacifierscontaining aluminum oxide and phosphorus pentoxide. These specialopacifiers are synthetically prepared from materials providing aluminumoxide and phosphorus pentoxide by calcining such materials with theproduction of an end product which when finely ground and used as a milladdition increases the reflectance value of the finished enamel. InSerial No. 629,912 filed November 20, 1945, by William J. Baldwin, nowPatent No. 2,500,231, there is discussed an enamel opacifier containingzirconium oxide, phosphorus pentoxide and aluminum.

In accordance with the present invention the special mill additionop'acifier consisting substantially entirely of phosphorus pentoxide andaluminum oxide may be prepared by suitably combining materialscontaining these materials or by combining the oxides themselves undersuitable calcining conditions. Ihus, the opacifier may be prepared froma mixture of suitable raw material containing the oxides mentioned, orof the oxides themselves, by calcining such mixture at temperaturesabove about 1600 F. and generally in the neighborhood of 2,00(l F., orat such other temperature at which ceramic combination of the compoundsoccurs to produce a relatively homogeneous sinter. The time requiredwill be from about hour to about 2 hours. After calcining the batch, theresultant product is thoroughly pulverized to an appropriate finemess,that is, on the order of /2 to 1 micron, and preferably as near thelower limit as: possible. The product so prepared, when used to theextent of about 24% as a mill addition to acornmercial zirconiumopacified rrit of the type developing opacity during the enamelingcycle, increased the reflectance value of the final enamel as comparedto the reflectance value of an enamel obtained without the presence ofsuch mill addition material. In general, the composition of the milladdition opacifier of the present invention comprises the binary systemaluminum oxide and phosphorus pentoxide within the range 59 to 75%A1203, 50 to 25% P205. One of the resultant opacifier compositions inthis range is a product with a calculated molar composition ofapproximately 3 AlaOsPzOs.

Where the pure oxides are not employed as raw materials from which themill addition of the present invention is manufactured, accepted rawmaterial used in the manufacture or enamel frits will be found suitable,that is to say, amblygonite, aluminum oxide, aluminum metaphosphate orother accepted sources of raw material.

As specific examples merely illustrative of the invention but notlimitative thereof, raw batches were made as shown in the followingexamples by grinding the materials of the raw batch together and heatingthe batch to about 2000 F. for one hour. The product of the calcinationwas then ground by wet milling the same until a particle size of aboutl'micron or less was obtained. Two parts of the resulting groundopacifier was added for each 100 parts of frit and milled therewith, thecomposition in the mill being as follows: a

100 parts frit 6 parts micronized clay 2 parts special opacifier 0.1part sodium nitrite 42 parts water The mixture in the mill was wetground to a fineness of 3 to 4% residue on a 200 mesh screen.

The resulting slurry was applied to 4" x 6 standv given in Table I andthe reflectance values are given in Table II.

Table I P t P t i Sample 3 I Composition-Parts by weight A 68 32 AluminaMetaphosphato-39,

Alumina Hydrate93. B l 75 Amblygonite-535, Alumina Hydrate87.5. C 50 50Amhlygonite-l07, Alumina Hy- I (Irate-22.

Table II Reflectance Values Application Wt., grams/ Composition sq. ft.Frit No. Opa0il A2% B- 2% O-2% lier 70. 2 70. G G8. 0 (i6. 4 I 40 74. L74. s 75. e 73. 2 l 5O 78. 5 77. e 79. 2 76. 2 i 60 so. 0 so. 3 79. 47s. 6

The milled zirconium opaoified frit having the special mill additionopacifier of the present invention added to it produced an enameledsurface having a considerably greater reflectance at the same rate ofapplication than the frit itself as set forth above in Table II. Thecommercial zirconium opacified frits of the type which develop opacityduring the enameling cycle and to which the mill addition opacifiers ofpresent invention may be added with advantage may possess the followingapproximate composition:

. Fer cent SiOz 25- i0 A12 03 7-16 B203 10-20 KNaO 10-16 CaO 3 7 F2 5-10ZrOz 10-15 ZnO 2-14 P205 0.5- 3 TlOz- O- 2 The preferred composition ofthe special mill addition opacifier of the present invention is Withinthe range %75% A1203 and 50%25% P205. calcined compositions consistingof A1103 and P205 Without this range do not produce acceptablereflectance values probably by reason of increased solubility of such anopacified composition in the frit when heated during the enameling. Itwill be understood that the usual amounts of clay, electrolyte and thelike are added in the mill in the usual way when preparing the enamelslurry.

What is claimed is:

' l. Th method of increasing the opacity of an enamel frit compositionof the zirconium opacified type wherein the opacity is developed in theenameling cycle which comprises adding to the enamel as a mill additionopacifier 2 to 4% by weight of a finely ground calcined materialcontaining 50 to aluminum oxide and 50 to 25% phosphorus pentoxide asthe essential ingredients.

2. A calcine suitable for use as a addition opacifier for enamel fritsof the zirconium opacified type which develop opacity on heating, whichcontains 50 to 75% aluminum oxide and 50 to 25% phosphorus pentoxide asthe essential ingredients.

mill

WILLIAM J. BALDWIN.

Name Date Elumenberg Dec. 27, 1927 Number

1. THE METHOD OF INCREASING THE OPACITY OF AN ENAMEL FRIT COMPOSITION OFTHE ZIRCONIUM OPACIFIED TYPE WHEREIN THE OPACITY IS DEVELOPED IN THEENAMELING CYCLE WHICH COMPRISES ADDING TO THE ENAMEL AS A MILL ADDITIONOPACIFIER 2 TO 4% BY WEIGHT OF A FINELY GROUND CALCINED MATERIALCONTAINING 50 TO 75% ALUMINUM OXIDE AND 50 TO 25% PHOSPHORUS PENTOXIDEAS THE ESSENTIAL INGREDIENTS.